In semiconductor fabrication process, various techniques of etching resist-imaged photomasks, silicon wafers or other semiconductor materials have been used. A wet etching technique conducted in an immersion tank is generally a practical high-throughput, flexible fabrication process. By properly selecting etchant chemicals, etch reactions with a target film are thermodynamically favored over reactions with other films. Desirable etch-rate ratios can be usually obtained.
A wet etching method is especially suitable for the blanket etching of materials such as polysilicon, oxide, nitride and metal. The method is capable of providing a desirable etch selectivity, a damage-free interface and particle-contamination-free wafers. In the more recently developed wet etching technology, automated robotic handling systems and ultra-pure chemicals are used to further improve particle control and process consistency. A well-controlled wet etching technique is therefore the choice of etching process in VLSI and ULSI fabrication techniques.
One of the key criteria in carrying out a wet etching process is that the etch products must be soluble in the etchant solution and therefore, no contaminating particles are generated. In an immersion etching process, the volume of the etching tank should be large enough to create enough pressure on the wafer surface in order to ensure an accurate balance of the etchant components; to keep the concentration of the etchant relatively constant; and to minimize the number of times the etchant tank must be changed. An etchant bath change creates expensive down time, and furthermore, the handling of highly hazardous corrosive materials creates potential safety problems.
Wet etching is a frequently used technique for stripping photoresist films from silicon wafers where the complete removal of resist images without adversely affecting the wafer surface is desired. The resist layer or images should be completely removed without leaving any residues, including contaminant particles that may have been present in the resist. The underlying surface of the photoresist layer should not be adversely affected, for instance, accidental etching of the metal or oxide surface should be avoided. Liquid etchant strippers should produce a reasonable bath yield in order to prevent redeposition of dissolved resist on the wafers. The etchant should completely dissolve the photoresist layer in a chemical reaction, and not just lifting or peeling so as to prevent redeposition. It is also desirable that the etching or stripping time be reasonably short in order to permit high wafer throughput.
Wet etchants such as sulfuric acid (H.sub.2 SO.sub.4) and mixtures of H.sub.2 SO.sub.4 with other oxidizing agents such as hydrogen peroxide (H.sub.2 O.sub.2) are widely used in stripping photoresist or in cleaning a wafer surface after the photoresist has been stripped by other means. For instance, a frequently used mixture is seven parts H.sub.2 SO.sub.4 to three parts 30% H.sub.2 O.sub.2, or a mixture of 88% sulfuric acid and 12% nitric acid. Wafers to be stripped can be immersed in the mixture at a temperature between about 100.degree. C. and about 150.degree. C. for 5.about.10 minutes and then subjected to a thorough rinse of deionized water and dried in dry nitrogen. This type of inorganic resist strippers, such as the sulfuric acid mixtures, is very effective in the residual-free removal of highly postbaked resist. They are more effective than organic strippers and the longer the immersion time, the cleaner and more residue-free wafer surface can be obtained.
In a typical wet chemical process tank, in order to ensure a perfect mix of acid and other components or a good uniformity in the acid itself, constant stirring or agitation of the solution in the tank is desired. While mechanical stirring or agitation techniques have been used, the moving components of a stirrer or agitator frequently generate contaminant particles that are detrimental to the wafer surfaces. Furthermore, a mechanical stirrer or agitating device tends to work only in a localized area of a large chemical tank while leaving a large area of the tank unmixed. Since wafers are usually loaded into a wafer boat and positioned at or near the center of the tank, too vigorous stirring or agitation of the liquid in the tank may cause the wafers in the wafer boat to move and as a result, stick to each other. Such wafer sticking problems result in uneven and unsatisfactory etching on the wafer surfaces.
An improved mixing method for liquids in a wet chemical tank is to force a fluid to enter the wet chemical tank at the bottom of the tank and then forming gas bubbles to rise from the bottom to the top of the tank and thereby carrying out mixing function along the way. The fluid used is normally a substance that readily reacts with the liquid in the tank such that either a gas or a vapor is generated to form the bubbles. One of such suitable chemical reactions occurring between a liquid in a tank and a fluid pumped therein is an acid and water. For instance, when phosphoric acid (H.sub.3 PO.sub.4) and ionized water are mixed in a tank, water reacts with the strong acid violently and immediately boils to form water vapor in bubbles and then the bubbles rise to the top of the liquid surface in the tank. During the process where the bubbles rise to the top of the liquid surface, the bubbles cause disturbance in the liquid and thus improve its uniformity by the agitation occurring in the liquid. A conventional system of a wet chemical process tank equipped with a fluid dispensing member, which is commonly called a spray bar, positioned at the bottom of the tank is shown in FIG. 1.
FIG. 1 shows a wet chemical process system 10 including a process tank 12 formed by a bottom wall 14 and four sidewalls 16 which are joined perpendicular to the bottom wall 14 forming a fluid-tight enclosure. The bottom wall 14 and the sidewalls 16 can be constructed of a corrosion-resistant material such as panels of stainless steel or steel panels coated with a corrosion resistant material such as Teflon. Due to the extreme corrosive nature of the processing liquids that are normally used in the wet chemical processing of semiconductor wafers, a securely sealed enclosure is a critical requirement for both maintenance and safety. The size of the tank 12 should be sufficiently large such that, for instance, two wafer boats (not shown) which are capable of holding twenty-four wafers each can be placed in the center of the tank while leaving an adequate space between the wafers and the walls such that a constant movement in the processing liquid can be achieved. Into the wet chemical process tank 12, a fluid dispensing member, or a spray bar, 22 is positioned at the bottom of the tank. A plurality of openings 24 are formed in the top surface 26 of the dispensing member 22 for allowing a fluid to enter the tank.
An enlarged, cross-sectional view of the dispensing member 22 taken at an opening 24 is shown in FIG. 1A. An internal fluid passage 28 is provided in dispensing member 22 which is in fluid communication with conduit 32 through a connector 34 and an elbow section 36 in the dispensing members 22. A precision volumetric pump 42 is used to pump a fluid such as deionized water from an inlet 44 through conduit 32 into the fluid passage 28. Since the volume of the deionized water that enters the process tank for reaction with phosphoric acid must be precisely controlled such that just the right amount of bubbles are generated, the pump 42 utilized must be a high precision volumetric pump that is capable of precisely controlling the amount of liquid pumped therethrough. For instance, into a normal wet chemical process tank of 20-liter capacity that is filled with phosphoric acid, only 40 cubic centimeter of deionized water should be pumped into the tank per minute. At such precisely controlled flow rate, just the right amount of water vapor in bubbles can be generated to suitably mixing the phosphoric acid contained in tank 12.
The fluid dispensing member 22, due to its extended exposure to strong acids when immersed in a tank, must be made of a corrosion-resistant material. A suitable material is Teflon which can be easily machined to provide the internal fluid passage 28 and the openings 24. The connector 34 and the conduit 32 should also be made by a corrosion-resistant material due to their prolonged exposure to corrosive liquids.
In a conventional wet chemical process tank, the fluid dispensing member, as shown in FIG. 1, has fluid outlets facing upward for easier exit of the fluid. Since wafer boats are normally positioned at the center of the tank and therefore, directly above the fluid dispensing member and are in the path of the bubbles formed by water vapor exiting the openings in the dispensing member. This can cause various processing problems. For instance, the bubbles can sometimes stick to a wafer surface such that the specific surface area can not be wetted by the etchant. This can lead to uneven etching of a wafer surface. The rising bubbles can also cause disturbance or movement of the wafers and thus causing the wafers to stick to each other. When the wafer sticking problem occurs, the wafer surfaces which are stuck together are not exposed to etchant solution and thus, poor etching occurs on the surfaces as a result. Furthermore, when the wafers are stuck together due to excessive disturbance from the rising bubbles, the temperature of the etchant liquid in the area immediate adjacent to the stuck wafers differs from the rest of the tank which also causes uneven etching at such local areas. This further creates non-uniformity of etching on the wafer surfaces.
It is therefore an object of the present invention to provide a wet chemical process tank for processing semiconductor wafers that does not have the drawbacks or shortcomings of a conventional wet chemical process tank.
It is another object of the present invention to provide a wet chemical process tank for processing semiconductor wafers that does not cause uneven etching of the wafers and requires only minor modifications in the equipment.
It is a further object of the present invention to provide a wet chemical process tank for processing semiconductor wafers that does not cause non-uniform etching of the wafers by preventing bubbles from a fluid dispensing member positioned at the bottom of the tank from directly contacting the wafers.
It is another further object of the present invention to provide a wet chemical process tank for processing semiconductor wafers equipped with a fluid dispensing member positioned at the bottom of the tank which allows a fluid to be dispensed only from one or both sides of the member such that bubbles formed do not contact the wafers directly.
It is yet another object of the present invention to provide a wet chemical process tank for processing semiconductor wafers that is equipped with a fluid dispensing member positioned at the bottom of the tank which dispenses bubbles from the vertical sides of the member to achieve uniform mixing of a liquid etchant while not directly disturbing the wafers.
It is still another object of the present invention to provide a fluid dispenser for use in a wet chemical process tank that is equipped with a fluid dispensing member having an elongated body, a generally rectangular shaped cross-section and a fluid passage contained therein in fluid communication with a plurality of openings provided on the two vertical sides of the elongated body.
It is another further object of the present invention to provide a fluid dispenser for use in a wet chemical process tank that is equipped with at least one support member of elongated shape connected to at least one end of a fluid dispensing member to support and stabilize the member when fluid is pumped through a fluid passage and a plurality of openings to enter the tank.
It is still another further object of the present invention to provide a method of treating semiconductor wafers in a wet chemical process tank by providing a fluid dispenser for positioning at the bottom of the tank which has an elongated body containing a fluid passage therein in fluid communication with a number of openings provided in at least one vertical side of the elongated body such that fluid may only enter the tank in a horizontal direction.